1. Field of the Invention
The present invention relates to a data flow amount control device and a data flow amount control method.
2. Description of the Related Art
In mobile communication systems such as cellular phones which have been widely used, services are provided with an entire service area divided into radio zones, which are referred to as cells. As shown in FIG. 1, such a mobile communication system includes a plurality of base stations covering cells, a plurality of mobile stations which perform communications by setting radio channels between the mobile stations and the base stations, and control stations which control the plurality of base stations and the mobile stations. Further, the control station is connected to a core network including an exchange and the like.
In such a configuration, signal transmission in the communications between the core network and the mobile station is performed through the control station and the base station. Thus, a base station performs relaying between a wired section and a radio section, the wired section being established between a core network and a base station via a control station, and the radio section being established between a base station and a mobile station.
The base station has a function of temporarily buffering a signal transmitted from the core network and control station. An object of the buffering function is to buffer a signal from the wired section when a transmission rate in the wired section is greater than that in the radio section.
Meanwhile, a high speed downlink packet transmission, which is of high-speed and large-volume, such as High Speed Downlink Packet Access (HSDPA) or 1× EV-DO is standardized, and also has been developed (for example, 3GPP TR25.848 v4.0.0 and 3GPP2 C. S0024 Rev. 1.0.0).
A main characteristic of such a high speed packet transmission scheme is to utilize an adaptive modulation and coding scheme in which packet transmission formats are adaptively changed according to radio conditions of a mobile station which performs a packet transmission. Here, the packet transmission formats are a data size, a modulation scheme, the number of codes, a coding rate of a packet and the like.
In this adaptive modulation and coding scheme, a radio transmission bandwidth changes with time according to a radio condition of a mobile station. For example, in the HSDPA, the data size of a packet, a modulation scheme, and the number of codes of a packet are controlled according to a radio condition between a mobile station and a radio base station, whereby an adaptive modulation and coding is performed.
Further, the above-described HSDPA or 1× EV-DO is a system in which a single radio transmission bandwidth is shared by a plurality of users, and the plurality of users share the single radio transmission bandwidth through time multiplexing, users multiplexing at the same time, or the like.
Further, the base station controls the order of transmission of packets to a plurality of mobile stations based on the instantaneous radio quality of each mobile station. Thereby, it is possible to increase throughput, that is, so-called system capacity, provided by the base station as a whole. Such a transmission order control of the packets by a radio base station is referred to as scheduling, and applying the scheduling to a packet transmission enables to increase communication capacity.
In this manner, when the single radio transmission bandwidth is shared by the plurality of users and when the scheduling is performed in the base station, a radio transmission bandwidth for each of the plurality users changes with time according to the number of the plurality of users and the way of the scheduling in the base station.
By the way, in a case of the mobile communication system configured as described above, when the transmission bandwidth of the wired section is larger than that of the radio section, packets tend to be stored in a temporal buffer of a relaying base station; and, when the transmission bandwidth of the wired section is smaller than that of the radio section, packets tend to be exhausted in the temporal buffer of the relaying base station.
Due to the limitation of the amount of the packets stored in the temporal buffer of the relaying base station, and the data overflowed from the buffer are deleted. In addition, due to the exhaustion of packets stored in the buffer of the relaying station, an entire radio bandwidth cannot be always used up.
Accordingly, when there is a difference between a transmission bandwidth in a radio section and a transmission bandwidth in a wired section, transmission efficiency is more likely to deteriorate due to the deletion of packets caused by a buffer overflow, or utilization efficiency of a radio transmission bandwidth is more likely to be reduced due to the exhaustion of the buffer.
In order to avoid the above two problems, it is necessary to control the transmission bandwidth in the radio section and the transmission bandwidth in the wired section so that these transmission bandwidths can be made as equal as possible.
From the viewpoint of suppressing the deletion of packets in a base station in communications between the core network and the mobile station, a method has been proposed in which an amount of data flow from a control station to a base station is controlled according to an amount of data stored in the buffer of the base station (Japanese Patent Application Laid-open Publication No. 2002-077987). Moreover, another method (Japanese Patent Application Laid-open Publication No. 2005-057323) has been also proposed for such a communication system in which a radio bandwidth greatly changes like a high speed packet communication scheme, and
This method makes it possible to prevent deteriorations in use efficiency of a radio bandwidth and in transmission efficiency of a system, although the use efficiency deterioration might be caused by exhaustion of a buffer and the transmission efficiency deterioration might be caused due to an execution of a retransmission control required to cope with the destruction of a signal attributable to buffer overflow. Using this scheme, the amount of inflow data to the base station can be controlled according to a data amount stored in the buffer, so that a buffer overflow in the base station can be avoided.